Believe it or not, but any business that supplies parking facilities for patrons have a duty and responsible for ensuring that the area being provided is fit for purpose. Erecting a sign which states that vehicles are left at the owners own risk does not hold any water in a court of law, and the land owners still have responsibility to the users as long as there is evidence to support any complaint.
Now wouldn't it be great if all of us car owners who have had their cars damaged due to having to use parking spaces that were too small to ensure their safe keeping, sent in multiple claims. do you think they would get the message, in reality I think not, but it would at least allow us to let off steam.
Believe it or not there is a UK 'standard' width laid down in law for parking spaces! And it hasn't been changed since it was introduced in the 1950s even though cars have grown so much. Believe it's about 1.8 metres, which is narrower than many cars, esp 4x4s!
So as long as Sainsbury's (or whoever the car park belongs to) have followed this width then no-one would get anywhere trying to prove in law that they hadn't carried out their statutory duty of care. Sorry!